The present invention relates to general to systems, devices and apparatus for heating liquids. More particularly, the present invention relates to an apparatus for heating electrically resistive liquids by passing electrical current through the liquid itself.
In this type of apparatus, the liquid because of its chemical composition, impurities or other physical characteristics functions as a resistive body capable of passing electrical current between two oppositely charged electrodes. The resistivity of the liquid transforms the electrical energy into heat which in turn increases the temperature of the liquid.
Tap water or in general water having sufficient mineral content to conduct electrical current is one of the liquids which is particularly suited for this type of heating. Theoretically, heating tap water by this electrical process is a very efficient technique because most of the input electrical energy is dissipated directly in the water in the form of heat. Furthermore, heating of the water may be performed instantly as it flows from the tap, eliminating or in some cases advantageously complimenting large, hot water storage tanks. While a conventional electric or gas hot water storage tank is limited in its ability to supply unlimited hot water during intervals of heavy use, the electrical resistance type of heater is capable of continuously supplying hot water or other heated fluids indefinitely.
Although generally this type of heating has been known for many years, none of the heretofore proposed devices have been as commercially successful as would be expected from the theoretical advantages of this type of liquid heating. Examples of prior art devices utilizing this heating principle are found in U.S. Pat. Nos. 2,529,688; 2,618,732; 727,361; 2,403,334; 2,572,337 and 2,748,253. Even though these and other electrical heaters operating on the liquid resistance principle have been proposed, it does not appear that any of these devices have enjoyed significant commercialization. It is believed that one reason for the lack of instant success of these devices can be attributed to several significant technical problems encountered in their operation.
The principle among these difficulties is the nonuniformity and variability of the conductivity or resistivity of the liquid. In particular, tap water varies in mineral content and thus conductivity depending upon the source of the water. Moreover, the conductivity of a given source of water may vary from time to time, thus resulting in a variable conductivity. Usually, two or more oppositely poled electrodes are used to provide a flow of current through the water flow. As the conductivity increases for example, the current flow also increases and the water for a given electrode voltage may become excessively hot. Additionally, excessive current may flow, overloading the power source.
A sudden increase in the water conductivity can result in instant scalding temperatures thus making the device potentially dangerous for ordinary household use. Although attempts have been made to solve this problem of varying water conductivity, none of these attempts have resulted in a practical, commercially feasible solution.
Another typical shortcoming of prior devices has been their inability to respond with sufficient speed for controlling the temperature level of the flowing water or other liquid. It will be appreciated that the water may be flowing through the heating device at a rapid rate and because of this it is necessary for the system to rapidly respond to the instantaneous temperature of the flowing water. In other words, the electrical energy supplied to the electrodes for heating the water must be highly responsive to the water temperature in order to add the necessary heat to the continuously flowing stream. Without such responsiveness, the system may result in hot and/or cold spots of water flow which detract from the performance of the device.
Still another technical problem which heretofore has not been adequately solved is found in the difficulty of switching the large currents required for heating the liquid. As an example of this, approximately 80 to 140 amperes of current may be required for raising the temperature of ambient tap water to a customary "hot" water temperature. At these current levels, it is difficult to quickly control the amount of electrical energy applied to the electrodes of the device. Simple switching devices are not satisfactory because of their propensity to arc at these current magnitudes. Even with the use of solid state switching devices, abrupt initiation and termination of such large current flows leads one to believe that a practical, economical, long lasting device operating under these requirements is not foreseeable from the technology heretofore developed on heaters of this nature.
It is an object of the present invention to provide an electrical apparatus for heating resistive liquids which solves the above and other problems encountered in the construction of a practical and commercially acceptable heating system.
This and further objects and various advantages of the electrical apparatus for heating resistive liquids according to the present invention will become apparent to those skilled in the art from a consideration of the following detailed description and appended drawings of an exemplary embodiment thereof.